Compounded rubber stock



Patented Oct. 26, 1954 GOMPOUNDED RUBBER s'rocK Alphonse Pechukas, Akron, Ohio, assignor, by

mesne assignments, to

Chemical Corporation,

poration of Delaware Columbia- Southern Pittsburgh, Pa., a cor- No Drawing. Application March 10, 1950, Serial No. 149,027

8 Claims.

The present invention relates to a novel composition of matter suitable for use as an improved rubber reinforcing pigment and also to rubber compositions containing such pigment and to the methods of producing these materials.

It is known that silica in finely divided state having a particle size below 0.1 micron is an effective rubber reinforcing pigment. However, one of the objections to finely divided silica when incorporated in rubber compositions resides in the fact that the compositions containing such silica cure at a very slow rate. Furthermore, the finally cured compositions often exhibit unduly low strength characteristics. In addition, rubber compositions containing finely divided silica tend to exhibit substantial heat build-up when the cured rubber composition is subjected to dynamic tension.

To a substantial degree, these objectionable characteristics of finely divided silica are avoided when the silica contains a substantial quantity (2 to 10 percent by weight) of free watenthat is water which is capable of being vaporized or driven off from the pigment by heating at a temperature of 105 C. for a period of. 24 hours. However, the required concentration of free water is not present in most silica pigments and in any event may be driven off during milling thus making control diflicult.

According to the present invention, I have been able to eliminate or substantially minimize these undesirable characteristics of silica and to provide a novel silica rubber pigment composition and novel rubber compositions containing such silica pigment which cure rapidly and which have optimum tensile strength and which do not exhibit the undesirable heat build-up commonly observed in prior silica pigments. I have found that these undesirable strength properties and heat build-up characteristics of silica may be avoided by coating the finely divided silica with a water miscible polyhydric alcohol or by incorporating silica and the polyhydric alcohol in a rubber composition. Thus, the silica may be precoated with the alcohol or uncoated silica together with gly- (301 or other polyhydric alcohol maybe compounded with rubber and the various other agents compounded with rubber.

Various polyhydric alcohols are suitable for the purpose. These alcohols should be water miscible and usually contain no more than 8 carbon atoms. Typical alcohols which have been found to be suitable include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropyleneglycol, propylene glycol, glycerol, methyl glycerol, trimethylene glycol, tetramethylene glycol, hexamethylene glycol, pinacol, styrene glycol and the butylene glyco'ls. Higher alcohols may be used provided such alcohols are water miscible.

The amount of glycol 01 other polyhydric alcohol which is used is capable of some variation. Normally, about 2 to 15 percent of the alcohol based upon the weight of the silica, is found to be suitable. While some departure from this range is possible, it is normally found that excessive amounts of alcohol have an adverse effect upon the properties of the ultimate rubber composition whereas lower concentrations of alcohol are insufiiciently efiective.

The use of polyhydric alcohol as herein contemplated has been found to be advantageous in connection with the calcium oxide silica pigment which has been described and claimed in a copending application of Edward M. Allen, Serial No. 63,205, filed December 2, 1948. Such application describes a method of preparing a silica pigment by reacting finely divided precipitated calcium silicate having an average ultimate particle size as determined by the electron microscope below about 0.1 micron (normally about 0.025 micron as measured by the electron microscope) with an acid having an anion which forms a water soluble calcium salt. Typical acids which may be used for this purpose include hydrochloric acid, nitric acid, hypochlorous acid, hydrobromic acid, and acetic acid.

Sufficient acid is added to produce a slurry having a pH on the acid side, usually below about 4. Thereafter, the pH of the slurry is normally adjusted above '7, normally to about 7 to 8, and the precipitated silica is recovered by decantation and filtration and is dried in an oven at temperatures of to C.

The water content of this product normally is above 4 percent by weight, frequently being as much as 15 to 20 percent of the total weight of the product. Such a pigment contains a substantial quantity, usually within the range of 2 to 10 percent by weight of the pigment of free water as previously defined. The balance of the water present is in the form of bound water which is the amount of water driven off from a silica pigment of the type herein described by heating the pigment at ignition temperature, for example, 1000 to 1200 C. until no further water can be removed, minus the amount of free water in the pigment. Bound water which is present in concentrations ofabout 2 ;to:-l percent by weight of the-pigment, appears to be in chemical combination with the silica.

The resultant silica also contains small amounts of calcium or like alkaline earth metal, depending upon the nature of the silicate used in its production, metal content, computed asalkaline earth metaloxide normally is present in' arange of about to 3 percent by weight of the total pigment. On the anhydrous basis, it this range is somewhat higher.

on the anhydrous pigment ranges fro'mabout to 5 percent by weight. The following are typical analysesof silica pigments which may be prepared according 'to'the' above mentioned application: P

The following is'ia comp ete analysis of a silica pigment of the type contemplated which. is supplied to the trade on a commercialscale:

Loss at 105".C; s percent 6.62 Loss on ignition do 11.62 pH a v i 1 8.4

5.0 gramssample 95 H O, 2jdrop s saturated KCl solution.

Percentages in the-above tables arezbyyweight.

' As pointed out inthe above mentioned appliea tion, the presence of the free water in the silica materially increases the rate of cure of rubber compositions in which the silica is incorporated and also increases the tensile and other strengths of the rubber composition. 'I-Ience, Where t e will be understood that Thus the alkaline: earth metal oxide content of the silica, computed and, in generaL'thealkaline earth p with this invention include the free water content is high, the use of a polyhydric alcohol in conjunction with the silica in a rubber composition is not absolutely necessary although improved results are obtained when the polyhydric alcohol is used in such cases. Where the free water content is low or substantially absent, very poor results are obtained unless the polyhydric alcohol is used.

While the invention is capable of use in connection with the finely divided silica prepared according totheabove mentioned application, it is not limited thereto but may be extended to use in connection with other silica compositions which, on the anhydrous basis, contain in excess of percent by weight of Si02. Typical silica compositions which may be used in accordance finely divided silica prepared {by bu'rningethyl silicate or like silicic acid ester in air or by other method capable of producing finely divided silica. Such silicas produced at such elevated temperatures rarely if ever contain an efiective amount of free water. Consequently the advantageous eiiect obtained by recourse to the polyhydric alcohol as herein contemplated is more marked than is the case when thepolyhydric :alcohol is used in connection with the above mentioned Allen application... i

,The rubber compositions herein contemplated may beprepared simply by milling'silica, polyhydric alcohol, rubber and rubber vulcanization agents and ,otherhconven'tional compounding agents together. in a conventional manner and vulcanizing the resulting I product. concentra tions ranging from 5 to volumes of silica per. hundred volumes of rubber arefound to be suitable. The term rubber. as herein contemplated, is intended to include natural rubber andt he conventional synthetic rubbers such as. butaidiene-wstyren'e cop'olymers known as GR S rubber which are copolymers of 10 to 60 per cent by weight of styrene and 90 to 40 per.-.

centby weight of butadiene, butadiene-acrylm nitrile copolymers derived by copolymerization of was 90percent by weight of butadiene, 60' to 10 percent of acrylonitrile. neoprene rubber, isobutylene'. polymers andv copolymers of isobutylene with 0.01 to 4.0 percent by weightofjisoprene (basedupon the weight of isobutyl ene) orother,

elastomers prepared'by polymerization of. butadiene-1,3, isopren'e; piperylene, 2,3-dimethyl butaoliene, 2 -chlorobutadiene l,3 or other comparable polymerizable compoundalone or in admixture with one or more organic, monomeric or ethylenic compounds including acr'ylo'nitrile, isobuty lene,

vinyl chloride, vinyl .acetate, styrene, methyl methacrylate, methyl alphachloroacrylate, methylacrylat'e and the like. According to a 'further embodiment of this invention, the silica "may be precoated with the polyhydric, alcohol prior to its incorporation in a rubber'cornposition. This may be efiected, for

example, by spraying the dry pulverulent silica.

with polyhydric alcoholwhiletumbling the silica or otherwise agitating it to an extent necessary in order to insure thorough mixing of the poly hydric alcohol with the silica. Such precoating offers certain definite advantages. In the first place, it permits preparation'of a standardized product which may be added to conventionalrub. ber compounds Without the necessity for adjust, ment of the rubber recipe by a, rubber com pounder 'during'the rubber compounding. Fur-.- thermore the polyhydric alcohol becomes more thoroughly associated witli-tliesilica and thus is more cifective in combating the disadvantageous properties ofthesilica than would be-the case where the alcohol simplyis' added to the rubber compound. V

The invention will be more fully understood by reference to the following illustrative examples:

EXAMPLE I streams of aqueous sodium silicate solution containing 100' grams per liter of SiO as Naz'O' (S102) 3.36- and calcium chloride solution containing 100 grams per user of caCl-ii'and- 30 to 40 grams per liter ofjs'o'dium chloride were feddis rectly into thecentral areapf a centrifugal at 150 F. Thdrates OT- floW were adjustedso that calcium chloride was approximately per cent in excess over' the s'toichioin'etric quantity required for reaction" and that the amount or liquid supplied to the pump" was about percent below the output capacity of the pump; Inc-onsequence, the solutions" were" subjected to turbulent intermixing in the pump;

The slurry of calcium silicate thus produced was introduced into a tank and suifi'cfieiit hyidfchloric acid solution containing 28% by weight' of HCI was added, with stirringyto reduce the pH of the slurry to 2. Thereup'or'i, sufiicint sodium hydroxide solution containing 40 byweight' of NaOH was added to raise the; pH of the slurry to 7.5. The precipitated silicawasrecovered by decantation and filtration and was dried in an oven at an oven temperature of 120 to-140" C. for 12 hours. The free water content of the product was within the range of 3 to 8 percent by Weight of the pigment.

Finely divided silica thus prepared; was dried at 105 C. for 24*hours in order to remove all of the free water. This material was ground in a micro} pulverizer and was placed in an oven at 105 C. for 48 hours. The finely divided silica thus obtained was placed in a conditioning room at a temperature of 25 'C. and a relative humidity of 50% for periods of time as indicated in- Table 1 below. Samples were removed from the conditioning room after times as indicated in'said Table I, the moisture content of eachsample' measured by drying a portion of the sample-at 105 C. for

24 hours and measuring the loss of free water" as defined and: each sample was compounded' in a rubber composition according to the following formula:

Parts" by weight GR-S 1 00 Zinc oxide 5 Sulfur 3 Phenyl-beta-naphthylamine 1 Benzothiazyl disulfide'n 1.20 Tetramethyl thiuram disulfid 0.15 Paracoumarone-indene resin 1 15.00 Diethylene glyco1 1 1 3.5 Silica pigment 57.00

The rubber compounds" so prepared were vulcanized according to standard accepted pro cedure and the vulcanized rubber products thus obtained were subjectedto tests according to standard acceptedmethods of testing well estab lished in the rubber artin order to determine the tensile strength, the 'tear strength and the 'opti mum rate of cure of the rubber compounds'ob= tained from each" ofthe samples produced 3.4" (Two hour conditioningi..- I

attests o The results of the teasers set forth in the following table.

Table: V

I Tensile Tear Free Moisture Content Percent .Gnreat by weight (based on initial 239 F., ggf ig Egg; pigment) Minutes inch thickness Under-cured. i l r 2,380 210 0.9 (No'conditioninfiln;

(Optimum Cure). 2.2 (One hour conditioning) I (Optimum cure)- -4 Optimum oure). l..

3.9 (4 hour conditioning);

A A A seessageeeesgseessagseaes These data illustrate the combined eifect of dihydric alcohol and? free water to increase the rate of cure of the rubber composition and. further to materially increase thetensile' strength of the composition at optimum curing time".

The" following testswer'e performed and demonstrate the results obtained when no dihyd'ri'c alcoholwa s present:

Finely divided silica-prepared according to the disclosure" of the abov'eidentified application" was dried in an oven at C'. overnight; The dried pigment contained 018 percent fre"water. 500 gram portions of this pigment were placed in pans and conditioned at 77 F3 and 50 percent relative huniidity'fdf-various periods of time as indicated in the tables below; The freewater content" of each ofth" cofid iti ofied sai fifils determined by drying'at- 105 C.--for 24; hours and each of: the conditioned'samples wasteland-sees a rubber composition according" to the following formula:

methods nonssnyusa in 'testiri, such-compost tions-andthe followingresultswere obtained:

Free Moisture Content Percent 'Ciue at Tensile, gfg by .we ehtl s 91. n t a F B l ads p r perm];

pigment) 2 square inch thickness .LyIt will benoted that the above tests clearly illustrate theadvantageous effect of the addition of jthe diethylehe glycol. Thus in Table I, it is shown that using 3.5 parts byweigh't'of diethylene' glycol a satisfactorily cured product was obtained within 2,0,to 3,0minutes even when the free water cohtent of theiproduct was as low as 0.9 and 2.2 percent respectively; In .contrast when no glycol was used,'as shown in Tame II,'"samp1es'"con'- taininglas muehasasi percent by weight of free water produced uncured rubber compositionseven after 60 minutes. Thus 'it will be seen that the presence of the diethylene glycol :lmoret. than doubled the rate of cure in this case. Similar results are obtained when ethylene glycol triethylene glycol or glycer l are used in lieu of the diethylene glycol. I I i 1 .Tozfurther,demonstrate theefiect,of diethylene glycol, silica prepared as in Examplel, andcone taming; 6.6 percent by weight of iree waterjand 4.8xpercent by weiglit of; bound Water, was com: pounded with GR-S rubber according -to the:following recipe testgspecimens upreparedtand cured and the vulcanized. rubber was tested to determine its: p y ical; prop s: d ng to; standard methods. The results, obtained wereras follows:

ismixe'd withee partsb weight (baseduponthe Weight of thepigment) of diethyl'en'e glycol; This coated product is milled with the rubbercompo sition' I sea-forth in Example I; omitting "the" diethylen glycol from the recipe; Results com parable to those set forth in Example I are obtained- Fig.2.

It will be understood that the rubber compositionshereincontemplated may contain numerous components in addition to silica and ,;rubber. Thus-a sh wn ath rexampl sthe i ion will; contain -the;.usua l vulcanizing compositions nc udin ac e e a or ,pla c ze ca on black a drqt er I p g entsv e c: i In: d al n lwi h G eS ru be l t-has be n.foundpadvant us to ino ra ebto-zfl,pa y weiehtiof a coumarone indene resin,,lpreferablyea; coumaroneresinhaving invention has beenide scribedwithreference'tp specific'detailsofcertain embodiments thereof it is notintended thatrsuch details shall be regardedaslimitationsflupon the scopeof; the invention except insofar as such limitations are, included tinithe accompanying claims.-

= iA th hethe present @Wheti c ai ed: 1

t1 rubber. composition whic h comprises. a rubber, finely, divided silica; containing not less than z nor ymorelthanj lo-percentby Weightof free-water, said v silica being in the proportion of 5 to lot)parts-byvyolume perlOOjpartsby wagear a rubber and, ,arwater; miscible poly: hydric alcohol-containing carbon= hydrogen and orgygen as itsqonly elementsdin substantial amount up $9 10 percent based uponthewcighllll of the silica. g

,.2 Avcomposition comprising a rubber finelyd'ivided' silica having an average ultimate particlejsiz'e below'aboutolmicronlin amounts rom aboutir5' to lOOfp'arts bylvolume per parts'oi a rubbenj an'd a substantial amount up to 15% by 'Weightof a water-miscible 'polyhyd'r'ic alcohol containing carbon,' hydrogen, and: oxygen ajsits only elements; based iiponthe weight or the silica m-rnemmmsamna p "3. A vulcanizatei of the compos'iti'onbf c1ai'm'2; '41 A:-composition"comprising'a rubber, finely divided silica havingvan avera'ge ultimate particlejsiz'elbelow" about 0.1 microns in amount from about 5 to 100 parts by volume percloo parts of a rubber," and. a substantial'amount upto 15 %.by i hio eth e elvcol a edmpont e wei ht of the silica in the composition. x

5 A composition comprising a rubber, finely- 'vid s a avin an av raeeult m t t arv-v ticle size below about 0 1 micron ina nount from about 5 to 100parts by volume per 100 parts of a rubber, and a substantial amountlupytol5% by weight of diethylerie glycol based upon the weight of the silica inthe composition". i j 6 v A composition "comprisingf a rubber, finelyg divided-silica having'an averageummate; particle size below about 0.1micron'in amount from about 5' to'lOO parts by'volume per 100 parts of a rubber, and a substantial amount up to 15l% by weight offglycerolbased' upon the fweightf'ofjthe silica in ei m -flj. 1 A cornposition" comprising a rubber, finely dividedjfsiilca having arr'ave'ra'ge ultiinate particle-size below 'about (lil'micron in arn ou ntfrolrn about-5 to IOO' partS by' volum'e per 100 parts'of arubber, and a substantial amount up 'to'' 15 by weight of propylene glycol based upon the weight; of:;the silica-in the composition;

8. A composition comprising a rubber, finelydivided silica having an average ultimate particle size below about 0.1 micron in amount from about 5 to 100 parts by volume per 100 parts of a rubber, and a substantial amount up to 15% by weight of triethylene glycol based upon the weight of the silica in the composition.

References Cited in the file of this patent UNITED STATES PATENTS Number Number Compoundin Particle Silica, Rubber World,

Name Date Almy Nov. 17, 1942 Sears et a1 Dec. 11, 1951 HER REFERENCES 

1. A RUBBER COMPOSITION WHICH COMPRISES A RUBBER, FINELY DIVIDED SILICA CONTAINING NOT LESS THAN 2 NOR MORE THAN 10 PERCENT BY WEIGHT OF FREE WATER, SAID SILICA BEING IN THE PROPORTION OF 5 TO 100 PARTS BY VOLUME PER 100 PARTS BY WEIGHT OF A RUBBER AND A WATER MISCIBLE POLYHYDRIC ALCOHOL CONTAINING CARBON HYDROGEN AND OXYGEN AS ITS ONLY ELEMENTS IN SUBSTANTIAL AMOUNT UP TO 10 PERCENT BASED UPON THE WEIGHT OF THE SILICA. 